Conventionally, there is known an airbag apparatus including an inflator that has an attachment/fixing flange member caulked or press-fitted thereto (see, for example, Patent Document 1). The airbag apparatus includes a cylindrical inflator that generates gas, an airbag that is inflated by the gas generated by the inflator, a flange member that has a flange surface radially extending outward from a side surface of the inflator, and a retainer having an insertion hole that allows the inflator to be inserted therein and serving as a base member to which the inflator is attached and fixed. The inflator is attached and fixed to the retainer by inserting bolts into a through-hole provided at a peripheral part of an opening of the airbag, a through-hole provided in the flange surface of the flange member, and a through-hole provided in the retainer and fastening nuts to the bolts.